What are the best tools atm to achieve this? Bot filters with captchas?
Using relays with paid gateways to become whitelisted. https://relay.nostr.com.au for example ain’t relaying Reply Guys.
Cool concept! Does that really keep out bots or just make it more expansive to run bot nets at scale?
Yes it keeps out the bots. They have to pay to have their npubs whitelisted and they ain’t doing that. Spam only works when its marginal cost approaches zero.